1. Field of the Invention
The present invention relates generally to a laser resonator, and more specifically to a stable laser resonator provided with an intracavity wavefront correction device. The correction device includes intracavity optics (optical elements) for correcting the phase of regenerative wavefront within the resonator cavity, in particular of solid-state lasers employing a lasing gain medium of a large cross-sectional area.
2. Description of the Prior Art
In an optical laser resonator, photons oscillating from one end of the resonator to the other end thereof constitute electromagnetic energy which forms an intense electromagnetic field. The shape of this field is precisely dependent not only upon the photon wavelength, but also upon the mirror alignment, curvature and spacing, and upon the bore diameter of the laser tube. This field can assume many different cross-sectional shapes, termed transverse electromagnetic modes (TEM), but only certain modes, or mixtures of them, are useful for utilizing the laser power.
To operate lasers in the fundamental (single) mode (i.e. TEM.sub.00 or Gaussian mode), an aperture is usually formed in the resonator to prevent oscillations of higher-order modes. Consequently, the laser efficiency is generally lower as compared with multimode operation (e.g. TEM.sub.11), because the gain medium (lasing medium) involved in laser action is small in volume. To construct high-output power lasers, the mode volume is required to increase. There are two schemes toward better solutions evolved mainly along two ways: stable telescopic resonators and unstable resonators.
In stable telescopic resonators, a magnifying telescope is additionally provided for a conventional stable cavity to expand the mode cross sectional area in the arm of the cavity where the electromagnetic field interacts with the gain medium. In the resonators of this type, the beam quality is high, but the TEM.sub.00 mode volume is still limited because of thermal distortion (i.e. aberration) induced in the gain medium. In addition, at high laser power, there arises another problem such that the optical elements may be damaged at the resonator position where the beam is small in dimension.
Unstable resonators are usually used when coupled to a gain medium of large volume with good optical quality, that is, with uniform azimuthal and radial modes without phase reversal in the output aperture. Although the output beam of an unstable resonator is a plane wave and has sufficiently low divergence, there exists a problem in that the electromagnetic field presents a peculiar spatial shape due to the usual diffraction-coupled output, which creates an annular near-field output beam, viz. a central hole in the near-field intensity distribution.
The configuration of the prior-art resonators will be described in further detail with reference to FIGS. 1 to 4 under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.